Doping of semiconductor materials with conductivity-determining type impurities, such as n-type and p-type elements, is used in a variety of applications that require modification of the electrical characteristics of the semiconductor materials. Boron is conventionally used to form p-type regions in a semiconductor material.
Some doping applications require patternized p-type (p+) regions to form advanced devices such as interdigital back contact (IBC), local back surface field (LBSF), or selective emitter (SE) solar cells. Other applications need global coverage such as for emitters for N-type (n+) solar cells or back surface fields (BSF) for p+ solar cells.
Boron-comprising dopants can be deposited to form specific patterns using application processes such as screen printing, spray application, spin coating, rotogravure application, inkjet printing, and the like. Screen printing involves the use of a patterned screen or stencil that is disposed over a semiconductor material. A liquid dopant is placed on top of the screen and is mechanically pressed through the screen to deposit on the semiconductor material (e.g. solar wafer). If the screen has a pattern formed by areas that have no pores and areas that do have pores, the material can be deposited in a pattern that corresponds to the pattern of the screen. Spin application involves spinning the semiconductor material at a high spin speed such as, for example, up to 1200 revolutions per minute or even higher, while spraying the liquid dopant onto the spinning semiconductor material at a desired fluid pressure. Spinning causes the liquid dopant to spread outward substantially evenly across the semiconductor material. The liquid dopant also can be sprayed onto a semiconductor material at a desired fluid pressure at a position substantially at the center of the semiconductor material. The fluid pressure causes the dopant to spread radially and substantially evenly across the wafer. Rotogravure printing involves a roller upon which is engraved a pattern. The liquid dopant is applied to the engraved pattern of the roller, which is pressed against a semiconductor material and rolled across the semiconductor material, thereby transferring the liquid dopant to the semiconductor material according to the pattern on the roller. Inkjet printing refers to a non-contact printing process whereby a fluid is projected from a nozzle directly onto a substrate to form a desired pattern. Each of the various application processes described above utilizes a dopant formulation with a viscosity and polarity suitable for the given process. For example, screen printing requires relatively high viscosity while inkjet printing requires a viscosity low enough so that the dopant can be dispensed from a nozzle.
Boron siloxane and boron silicate dopants are often used in the above-described application processes and are typically made by sol-gel processes using polymers and oligomers. However, these dopants suffer from a number of drawbacks. For example, when produced using a sol-gel process, such conventional materials typically do not exhibit relatively high viscosity. Because of the low viscosity, “thickeners” are required, which must be removed during subsequent processing leading to additional challenges. Further, the molecular weight of such dopants is unstable, increasing at room temperature. Accordingly, the dopants typically must be transported, used, and stored under refrigeration. In addition, the dopants can be formed with pockets or regions of high boron concentration or high silicon concentration that can adversely affect the material or electrical characteristics of the post-diffusion electrical devices. Moreover, fabrication of the dopants requires the use of solvents that can be volatile and flammable, thus requiring expensive engineering and safety controls. And the fabrication process utilizes polymers and oligomers mixed directly in a solvent (or solvent mixture) that is limited by its suitability for the polymerization reaction. Further, instability of the sol-gel material and volatility of the solvent could dry and clog the printing apparatus. For example, such drying and clogging can result in a short “pot life” of a screen printer. Typically, it is preferable in high volume manufacture processes to have a “pot life” of a screen printer of over eight hours to avoid changing screens between personnel shifts.
Accordingly, it is desirable to provide borate esters and boron-comprising dopants that can be formulated using polyol-substituted silicon monomers. It is also desirable to provide borate esters and boron-comprising dopants that can be fabricated with variable viscosities with molecular weights that can exhibit stability during use and storage. In addition, it is desirable to provide methods for fabricating boron-comprising dopants. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.